Method of separation of gaseous hydrocarbons from gasoline



N. H. SCOTT sept. 3o, 1969 METHOD 0F SEPARATION OF' GASEOUS HYDROCARBONSPROM GASOLINE Filed Nov.' 20, 1967 nk aux YN .s

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/0 A N mw United States Patent O U.S. Cl. 208-101 5 Claims ABSTRACT OFTHE DISCLOSURE Method for recovering gasoline and normally gaseoushydrocarbons from the effluent of a hydrocarbon conversion zone, such asa coking unit. The method utilizes a combination of low and highpressure separation Zones operating in conjunction with a dualabsorption zone system. Gasoline and LPG are recovered as separateproducts.

BACKGROUND O-F THE INVENTION This invention relates to a separationprocess. It also relates to a method for separating the efiluent from ahydrocarbon conversion zone into normally gaseous products and normallyliquid products. It particularly relates to a method for recoveringgasoline and normally gaseous hydrocarbons from the eiiiuent of ahydrocarbon coking unit.

It is yknown in the prior art to subject relatively heavyhydrocarbonaceous materials to heat soaking or thermal crackingconditions in order to convert the materials to coke and upgradedhydrocarbon products, such as LPG, gas oil, and gasoline.

The prior art processes of coking generally take the form of delayedcoking, fluid coking, etc. Since the standard coking processes are wellknown in the prior art, it is not considered necessary to show thecoking process in detail. The units presently available for cokingoperate by heating a feed to a temperature from 750 F. to 950 F. andunder a pressure of from about l0 to 100 p.s.i.g. Sufficient residencetimes from a few seconds to perhaps several hours areutilized to convertthe heated material to coke and lower molecular weight products. Thecoke is removed from the coking drums and the uid efiiuent is sent tohydrocarbon recovery facilities.

The prior art schemes for separating the eiuent `from a conversionprocess, such as a coking unit, include broadly the use of afractionation column, an absorption column, and a stabilizer column.Conventionally, coker gasoline, coker gas oil, and normally gaseoushydrocarbons are obtained in these prior art processes.

The present invention provides improvement over the prior art schemes byutilizing a combination of processing techniques which achieve asubstantially greater recovery of desired normally gaseous hydrocarbonsin high purity while at the same time maintaining the quality of thecoker gasoline at a desirably high level.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide `a method for separating the effluent of a hydrocarbonconversion zone into normally gaseous hydrocarbons and normally liquidhydrocarbons.

It is another object of this invention to provide an improved method forseparating the fluid effluent of a coking conversion zone into gasolineand normally gaseous hydrocarbons.

Therefore, the present invention provides an improved method forrecovering gasoline and normally gaseous hydrocarbons from the ffuidefiiuent of a hydrocarbon conversion unit which comprises the steps of:(a) frac- 3,470,084 Patented Sept. 30, 1969 ice zone a first rich oilfraction and a second gaseous stream;

(e) passing said second gaseous stream into a second absorption zonemaintained under absorption conditions including the presence of atleast a portion of said gas oil fraction as absorption medium; (f)removing from said second absorption zone a second rich oil stream and afirst product stream comprising C2 and lighter components; (g) passingsaid first rich oil into said separation zone of Step (b) and passingsaid second rich oil stream to said fractionation zone of Step (a); and,(h) introducing the remaining liquid stream of Step (b) into a secondfractionation zone and recovering therefrom a second product streamcomprising normally gaseous hydrocarbons and a third product streamcomprising gasoline.

Another embodiment of this invention includes the method hereinabovewherein a portion of said third product stream is passed into said firstabsorption zone as absorption medium.

Thus, it is seen from the broad embodiments presented hereinabove thatthe present invention provides a facile and economical separationprocess for recovering gasoline and normally gaseous hydrocarbons, suchas C3 and C4 hydrocarbons, from the eiuent of a hydrocarbon conversionzone. The critical features of the invention include, in combination, afirst fractionating column, a liquid-gas separation zone, dualabsorption zones, and a second fractionation zone.

It is to be noted that the present invention has been described as beingbroadly applicable to separating the fluid effluent from a hydrocarbonconversion unit. More particularly, however, the present invention isuniquely applicable to separating the fluid effluent from a hydrocarboncoking unit. However, the present invention is not to be limited to thecoking operation; rather, it is to be limited only by the scope of theappended claims since such separation scheme can be utilized to separateany uid effluent containing the various components to be separatedherein.

To produce a fiuid efiluent which is separated according to the practiceof the preferred embodiment of the present invention, a high boilinghydrocarbon charge, such as crude bottoms from the vacuum distillationof reduced crude, vacuum tar, reduced crudes, topped crudes, blendsthereof, etc. is charged into a conventional conversion operation whichis maintained -under conventional conversion conditions, such as cokingconditions of either the delayed type or uid type. The charge materialis first preheated and the preheated charge is fed into the bottomportion of, for example, a coker drum or plurality of drums which issubjected to elevated temperatures and usually moderate pressures. Undercoking conditions, destructive distillation of the charge materialoccurs and results in the formation of lighter boiling hydrocarbons andcoke. The lighter boiling hydrocarbons are withdrawn from the coker drumwhile the coke is usually caused to remain therein. When the desiredamount of coke is formed, the conventional manner of operation at thatpoint discontinues the introduction of the preheated charge material andthe deposited coke is removed from the drum. The lighter boilinghydrocarbon materials which have been withdrawn from the coker drumcomprise the preferred fluid effluent to be separated according to thepresent DESCRIPTION OF THE DRAWING Referring now to the drawing, asuitable high boiling charge material is passed Avia line intohydrocarbon conversion zone 11 which may be a conventional coking unit.Fluid eiiluent from the conversion zone is withdrawn via line 12 andpassed into fractionation zone 13. Suitable fractionation conditions aremaintained in zone 13 to separate the fluid efiiuent, initially, into alight distillate fraction which is Withdrawn via line 14, a side-cut gasoil fraction which is withdrawn via line 15, and a heavy gas oilfraction which is withdrawn from the lower portion of zone 13 via line16. Depending upon the characteristics of the fluid effluent in line 12,there may be more or less product streams withdrawn from zone 13.

The light distillate fraction containing gasoline boiling range materialand normally gaseous hydrocarbons such as C2, C3, and C4 hydrocarbonsadmixed with other gases, such as hydrogen, is passed via line 14 intofirst separation zone 17 which is maintained under relatively lowpressure. Suitable separation conditions are maintained in zone `17 inorder to separate the light distillate fraction into a first gaseousstream which is removed via line 18 and a first liquid stream which iswithdrawn via line 19.

The material in line 18 comprises a broad mixture of normally gaseoushydrocarbons, including the previously mentioned hydrogen and acidgases, if any, and is at a relatively low pressure. According to thepreferred embodiment of this invention, the first gaseous stream in line18 is passed into compressing means 20 for raising the pressure thereofto a relatively high level. The cornpressed gaseous stream is removedfrom compressing means 20 via line 18 (a), admixed with hereinafterspecilied liquid stream from line 21, and the admixture passed via line22 into separation zone 23 which is maintained under a relatively highpressure. Other suitable operating conditions are maintained in zone 23to separate the admixture into a second gaseous stream which iswithdrawn via line 24 and a second liquid stream which is withdrawn vialine 25.

The second gaseous stream is introduced via line 24 into firstabsorption zone 26, preferably, at the lower end thereof. Absorptionzone 26 is maintained under absorption conditions including the presenceof at least a major portion of the rst liquid stream withdrawn fromseparation zone 17 as at least one part of the absorption medium thereinand which is introduced into absorption zone 26 via line 19. As morefully discussed hereinafter, an additional part of the absorption mediummay also be introduced into absorption zone 26 via line 35. Otheroperating conditions are maintained in absorption zone 26 in order toabsorb substantially all of the C3 and C4 hydrocarbons as Well as someC2 hydrocarbons into the absorption medium. Thereafter, the first richabsorption oil s withdrawn from zone 26 via line 21 and passed intoadmixture with the compressed gaseous material from line 18(a), aspreviously mentioned.

A third gaseous stream comprising C2 and lighter gaseous componentscontaminated with C3 and C, hydrocarbons is withdrawn from absorptionzone 26 via line 27 and passed into second absorption zone 28. Theabsorption medium utilized in absorption zone 28 is a portion of thelighter gas oil stream which was removed from fractionation zone 13 vialine 15. This lighter gas oil absorption medium is passed via line 29into the upper portion of absorption zone 28 for contact therein withthe gaseous material being introduced via line 27. A first productstream comprising C2 and lighter components including hydrogen and acidgas, if any, is `withdrawn from absorption zone 28 via line 30 and,preferably, sent to fuel. The second rich absorption oil is withdrawnfrom absorption zone 28 via line 31 and returned to fractionation zone13, preferably, as reux therein at a locus above the withdrawal locusfor the lighter gas oil material in line 15.

Returning now to separation zone 23, the second liquid stream in line 25is now passed into second fractionation zone 32 which is maintainedunder fractionation conditions suflicient to produce a gaseous streamcomprising, preferably, C, hydrocarbons contaminated with C3 and C.,hydrocarbons which is withdrawn via line 33. It is distinctly preferred,in order to maximize the recovery of C3 and C4 hydrocarbons, that thematerial in line 33 be sent via line 37 into separation zone 23 inadmixture with the previously mentioned first rich absorption oil inline 21. However, if desired, another product stream comprising theselight hydrocarbons may be removed from the system via line 33. A secondproduct stream comprising C3 and C4 hydrocarbons is removed from zone 32via line 36 and a third product stream comprising a conversion gasolineis withdrawn from the system via line 34. As previously mentioned, it ispreferred that a portion of the third product stream in line 34 bepassed via line 35 into first absorption zone 26. However, a portion ofthe material in line 35 may also be sent to zone 13, or zone 18, or zone23, by means not shown, if desired. A still further preferred embodimentof this invention is characterized Iby having the material in line 35introduced into absorption zone 26 at a locus above the locus ofintroduction of the first liquid stream in line 19.

By operating in the manner set forth hereinabove it was found thatextremely high recoveries (i.e., quantity and quality) of the normallygaseous hydrocarbons, such as C3 and C4 hydrocarbons, was achieved. Inaddition, a desrably stabilized gasoline product and, in the preferredembodiment, two gas oil fractions, were obtained as products.

'It is further noted that the preferred embodiment is characterized byhaving the rich oil from the rst absorption zone being passed into thesecond separation zone maintained at relatively high pressure therebyproviding enrichment of the liquid with normally gaseous componentswhich are subsequently separated in fractionation zone 22. Similarly,the preferred embodiment is characterized by having not only twoabsorption zones, but by having also two distinct absorption mediumswhich are introduced into the first absorption zone. By operating inthis manner, excellent recovery of C3 and C4 hydrocarbons is obtained.

The description of the drawing has not included specific operatingdetails for each of the pieces of equipment contained in the combinationmethod. It is deemed within the skill of those experienced in this artto choose the proper operating conditions to effectuate the variousseparations required by the description of the invention. Thefractionation steps, the absorption steps and the separation steps arebasically conventional operations which have been combined in a novelmanner to produce an improved result. Those skilled in the art will alsorecognize that the required separating conditions will be, of course,influenced to a considerable extent lliy the characteristics of thefluid to be separated in line However, the following example isfurnished to set forth the best mode contemplated for practicing thepreferred embodiment of the invention.

EXAMPLE A commercial size coking unit was operated accordmg toconventional practice. The fluid eluent, after separation of the coke,was passed into apparatus schematically arranged as in the attacheddrawing.

The charge to the coking unit (line was a vacuum reduced crude havingthe following properties:

Gravity, API 19.11 Sulfur, wt. percent 0.16 Con Carbon, wt. percent 10.9UOP K factor 12.25

After suitable pre-heat, the feedstock Was charged at a rate equivalentto 6000 barrels per stream day into the coking zone at a transfertemperature of about 900 F. and a coke chamber pressure of about 75p.s.i.g. After separation of the coke, the fluid effluent was ultimatelypassed via line 12 into fractionation zone 13.

The iluid effluent or vapors from the cake chamber are passed into thelower section of fractionator 13 at a temperature of 820 F. and apressure of about 75 p.s.i.g. An overhead stream comprising normallygaseous material and gasoline is withdrawn via line 14 at a temperatureof about 302 F., a side-cut stream comprising light gas-oil is withdrawnvia line 15 at a temperature of about 585 F., and a bottoms streamcomprising heavy gas-oil is withdrawn via line 16 at a temperature ofabout 750 F. Under these conditions the following -product streams wereseparated (composition dla-ta are in mols per hour):

Line No. Line No.

14 Component Component Total 798. 56

The material in line 14 is cooled by condensing means, such as anair-iin condenser, to a temperature of about 100 F. and passed intoseparator 17 which is under a pressure of about 60 p.s.i.g. A gaseousstream is Withdrawn from separator 17 via line 18 and a liquid streamwithdrawn via line 19. These withdrawn streams had the followingcomposition (data are in mols per hour):

The gaseous stream in line 18 is passed into centrifugal compressormeans 20 wherein the pressure is raised from about 60 p.s.i.g. to about225 p.s.i.g. and then admixed, 'as previously mentioned, with recyclestreams from lines 21 and, preferably from line 37, respectively. Theadmixture is then introduced into separator 23 at a temperature of about100 F. and a pressure of about 215 p.s.i.g. Under these conditions agaseous stream is separated and withdrawn via line 24 and a remainingliquid stream is withdrawn via line 25. The composition of thesewithdrawn streams from separator 23 had the following composition (molsper hour):

Line No.

Component 22 24 25 The liquid stream in line 25 is introduced intofractionation zone 32 which preferably is divided into two separatedistillation columns, the iirst one being typically a gasoline stripperfor the removal of C2s and lighter material from the feedstock and thesecond one being a debutanizer column for the recovery of C3 and C4hydrocarbons and gasoline as product streams. For convenience sake,however, the dual fractionation zones have been shown in the attacheddrawing as a single column. By operating the fractionation zones underconventional conditions of temperature and pressure, an overhead streamcomprising the C2 and lighter components contaminated with C3 and C4hydrocarbons is withdrawn via line 33. In the preferred embodiment ofthe invention the material in line 33 is passed via line 37 and 21 intoadmixture with the `compressed vapors in line 18(a) as previouslymentioned. However, if desired, the material in line 33 may be Withdrawnfrom the system and no recycle to high pressure separator 23 beingemployed. Obviously, a small amount of material or large amount ofmaterial or no amount of material may be .Withdrawn from the system vialine 33. Typically, a gasoline stripper would operate with a ash zonetemperature of about F. and a pressure of about 242 p.s.i.g. Similarly,the debutanizer column could operate with a ash zone temperature ofabout 400 F. and a flash zone pressure of about p.s.i.g. Under theseconditions a stream comprising C3 and C4 hydrocarbons is withdrawn fromfractionation zone 32 via line 36 and a gasoline product stream iswithdrawn from the bottom of the debutanizer column via line 34. Asillustrative of the type of separation which may be accomplished infractionation zone 32, the following composition data is provided (molsper hour):

Line No.

C6533SF Total 11o. 02 96. 54 201. 09

Referring again to high pressure separator 23, the gaseous material inline 24 is passed into a iirst absorption zone 26 at a temperature ofabout 100 F. and a pressure of about 212 p.s.i.g. Sufficient operatingconditions are maintained in absorption zone 26 to dissolvesubstantially all of the C3 and C4 hydrocarbons into the lean oil whichcomprises at least in part the material in line 19 the composition ofwhich has previously been given. The lean oil is introduced intoabsorption zone 26 so that effective counter-current contact may beobtained between the liquid and vapor in the zone. If desirable ordeemed necessary, suitable packing material may also be placed in zone26, the type and quantity of which are well known to those skilled inthe art. Additionally, another portion of the absorption mediumcomprises gasoline from line 34 which is introduced into zone 26 vialine 35 (the composition of the material in line 34 has also beenpreviously given). Under the operating conditions imposed in zone 26 theC2 and lighter materials are effectively rejected from this zone and arewithdrawn from absorber 26 via line 27. The combined rich oil having C3and C., hydrocarbons absorbed therein is similarly withdrawn from zone26 via line 21, admixed with the material coming from line 37, andfurther admixed with the gas leaving compressor 20 via line 18(11) forrecycle and liquid enrichment in high pressure separator 23.Illustrative of the separation obtained in absorber 26 are the followingcomposition data (mols per hour):

The rejected C2 and lighter material contaminated with C3-icomponents ispassed via line 27 into second absorber 28 at a temperature of about 118F. and a pressure of about 205 p.s.i.g. The gas in line 27 is introducedinto the lower portion of absorber 28 and contacted in counter-currentmanner therein with lean oil introduced into the upper portion ofabsorber 28 via line 29. As previously mentioned, the absorber oil ormedium for use in zone 28 is a portion of the light gas-oil productwhich was removed from fractionating column 13 via line 15, thecomposition of which has previously been given. The C2 and lightermaterial is ultimately rejected via line 30 and sent preferably to fuel.The rich oil from second absorption zone 28 is withdrawn via line 31 andpreferably returned to the upper portion of fractionating column 13 asadditional reflux thereon. A typical composition data of the separationobtained in absorber 28 is shown as follows (mols per hour):

8 PREFERRED EMBODIMENT Therefore, in summary, the preferred embodimentof the present invention includes the method for separating the fluideffluent from a coking reaction zone which comprises: (a) passing saidefiluent into a first fractionation zone under conditions sufficient toproduce a first distillate fraction comprising hydrogen, normallygaseous hydrocarbons, and gasoline, a second distillate fractioncomprising light gas-oil, and a third fraction comprising heavy gas-oil;(b) introducing said first distillate into a first separation zonemaintained under conditions including relatively low pressure sufficientto produce a first gaseous stream and a first liquid stream; (c)compressing said first gaseous stream and admixing the compressed streamwith a hereinafter specified rich oil stream; (d) passing the admixtureof Step (c) into a second separation zone maintained under conditionsincluding relatively high pressure sutlicient to produce a secondgaseous stream and a second liquid stream; (e) passing said secondgaseous stream into a first absorption zone maintained under conditionssuficient to absorb Ca-ihydrocarbons into an absorption mediumcomprising at least in part said first liquid stream of Step (b); (f)introducing a third gaseous stream comprising C2 and lighter gaseouscomponents contaminated with C3. and C4 hydrocarbons into a secondabsorption zone maintained under conditions sufficient to absorb saidcontaminants into an absorption medium comprising at least in part aportion of said light gasoil from Step (a); (g) removing from saidsecond absorption zone a first product stream comprising hydrogen, andC2 and lighter hydrocarbons; (h) passing said second liquid stream intoa second separation zone under conditions sufiicient to produce a secondproduct stream comprising C3 and C4 hydrocarbons and a third productstream comprising gasoline; (i) introducing the rich oil containingabsorbed C3 and C4 hydrocarbons from Step (c) into admixture with saidcompressed gaseous stream as specified in Step (c); and, (j) returning aportion of said third product stream to said first absorption zone asanother part of said absorption medium.

The invention claimed:

1. Method for recovering gasoline and normally gaseous hydrocarbons fromthe fluid eflluent of a hydrocarbon conversion unit which comprises thesteps of:

(a) fractionating said efliuent in a first fractionation zone into alight distillate fraction and gas-oil fraction, said light distillatefraction containing gasoline and normally gaseous hydrocarbons;

(b) separating said light distillate fraction in a separation zone intoa first gaseous stream and a first liquid stream;

(c) introducing at least a portion of said first gaseous stream into afirst absorption zone maintained under absorption conditions includingat least a portion of said first liquid stream as absorption medium;

(d) removing from said first absorption zone a first rich oil fractionand a second gaseous fraction;

(e) passing said second gaseous stream into a second absorption zonemaintained under absorption conditions including the presence of atleast a portion of said gas-oil fraction as absorption medium;

(f) removing from said second absorption zone a second rich oil streamand a first product stream comprising C2 and lighter components;

(g) passing said first rich oil into said separation zone of Step (fb)and passing said second rich oil to said fractionation zone of Step (a);and,

(h) introducing the remaining liquid stream of Step (b) into a secondfractionation zone and recovering therefrom a second product streamcomprising normally gaseous hydrocarbons and a third product streamcomprising gasoline.

2. Method according to claim 1 wherein a portion of said third productstream is passed into said -iirst absorption zone as absorption medium.

3. Method according to claim 1 wherein said third product streamcomprises C3 and C4 hydrocarbons.

4. Method according to claim 1 wherein said hydrocarbon conversion unitcomprises a coking reaction zone.

5. Method for separating the fluid efliuent from a coking reaction zonewhich comprises:

(a) passing said etiiuent into a rst fractionation zone under conditionssufficient to produce a irst distillate fraction comprising hydrogen,normally gaseous hydrocarbons, and gasoline, a second distillatefraction comprising light gas-oil, and a third fraction comprising heavygas-oil;

(b) introducing said rst distillate into a irst separation zonemaintained under condition including relatively low pressure suicient toproduce a iirst gaseous stream and a first liquid stream;

(c) compressing said rst gaseous stream and admixing the compressedstream with a hereinafter specified rich oil stream;

(d) passing the admixture of Step (c) into a second separation zonemaintained under conditions including relatively high pressure suicientto produce a second gaseous stream and a second liquid stream;

(e) passing said second gaseous stream into a Ifirst absorption zonemaintained under conditions sufficient to absorb C3+ lhydrocarbons intoan absorption medium comprising at least in part said first liquidstream of Step (b);

(f) introducing a third gaseous stream comprising C2 and lighter gaseouscomponents contaminated with C3 and C4 hydrocarbons into a secondabsorption zone maintained under conditions sufficient to afbsorfb saidcontaminants into an absorption medium comprising at least in part aportion of said light gas-oil from Step (a);

(g) removing from said second absorption zone a first product streamcomprising hydrogen, and C2 and lighter hydrocarbons;

(h) passing said second liquid stream into a second separation zoneunder conditions sulicient to produce a second product stream comprisingC3 and C4 hydrocarbons and a third product stream comprising gasoline;

(i) introducing the rich oil containing absorbed C3 and C4 hydrocarbonsfrom Step (e) into admixture with said compressed -gaseous stream as`specified in Step (c); and,

(j) returning a portion of said third product stream to said -rstabsorption zone as another part of said absorption medium.

References Cited UNITED STATES PATENTS 2,182,536 12/1939 Eaton 208-1012,745,889 5/1956 Johnston et al. 208--101 2,908,625 10/ 1959 Melder etal 208--101 2,939,834 6/ 1960 Evans 208-101 2,985,583 5/ 1961 Gilmore208-101 HERBERT LEVINE, Primary Examiner U.S. C1. X.R.

